This work is designed to build on our growing understanding of the mechanism by which tau becomes abnormal in Alzheimer's disease, and the critical factors that lead to tangle formation and cell death. We now seek to test new specific hypotheses regarding the mechanisms responsible for these events. Tyrosine phosphorylation of tau seems to be a very early event in AD, and the kinases responsible for these phosphorylations will be a major focus of this application. Mechanisms that can activate these kinases will also be examined to determine if associations with pathology can be demonstrated. New data on an apparent AD-specific phosphorylation of APP and the binding of Pin1 have suggested approaches to examination of the relationship between changes in APP processing and tau pathology. The specific aims are: 1. We will test the hypothesis that phosphorylation of tau on Y18 and/or 394 are candidates for production of conformational changes in tau in AD. Fyn and abl are the kinases that may be responsible for these phosphorylations, and the association of these proteins with tau pathology will be examined in detail. 2. The hypothesis that Abeta deposition triggers activation of src family kinases (and tyrosine phosphorylation of tau) has been proposed by others, and this will be tested in a rigorous fashion in the AD brain. The accumulation of pY18, fyn and abl with Abeta will be examined by immunocytochemistry using the same tissues employed in Aim 1. The possibility that Abeta effects on tyrosine phosphorylation of tau might be indirectly mediated through microglial production of TNF alpha will also be examined. 3. The interaction of tau with fyn and abl will be further examined in cultured cells. Cells will be transfected with specific tau isoforms along with fyn or abl. The hypothesis that tyrosine phosphorylation of tau causes conformational changes that may be sensitive to tau isoform composition will be tested. 4. We will test the hypothesis that changes in APP processing can induce changes in tau conformation and phosphorylation in cell culture systems selected to have a variety of tau isoform compositions. 5. Pin1 binds to both phosphorylated tau and phosphorylated APP, and both sites are phosphorylated in the AD brain. The hypothesis that Pin1 is involved in conformation changes of tau and perhaps in modifying APP processing will be tested. This work will help understand the process of AD, and help lead to new therapeutic strategies.